Multispectral biometric imaging

ABSTRACT

Methods and systems are provided for performing a biometric measurement on an individual. A purported skin site of the individual disposed relative to a platen is illuminated under distinct optical conditions during a single illumination session. At least one of the optical conditions includes illuminating the purported skin site with light at an angle greater than a critical angle defined by an interface of the platen with an external environment. At least another of the distinct optical conditions includes illuminating the purported skin site with light at an angle less than the critical angle. Light scattered from the purported skin site is received separately for each of multiple optical conditions to derive a multispectral image of the purported skin site.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a nonprovisional of, and claims the benefit of thefiling date of each of the following applications: U.S. Prov. Pat. Appl.No. 60/576,364, entitled “MULTISPECTRAL FINGER RECOGNITION,” filed Jun.1, 2004 by Robert K. Rowe and Stephen P. Corcoran; U.S. Prov. Pat. Appl.No. 60/600,867, entitled “MULTISPECTRAL IMAGING BIOMETRIC,” filed Aug.11, 2004 by Robert K. Rowe; U.S. Prov. Pat. Appl. No. 60/610,802,entitled “FINGERPRINT SPOOF DETECTION USING MULTISPECTRAL IMAGING,”filed Sep. 17, 2004 by Robert K. Rowe; U.S. Prov. Pat. Appl. No.60/654,354, entitled “SYSTEMS AND METHODS FOR MULTISPECTRAL FINGERPRINTSENSING,” filed Feb. 18, 2005 by Robert K. Rowe; and U.S. Prov. Pat.Appl. No. 60/659,024, entitled “MULTISPECTRAL IMAGING OF THE FINGER FORBIOMETRICS,” filed Mar. 4, 2005 by Robert K. Rowe et al.

This application is related to copending, commonly assigned U.S. patentapplication Ser. No. 11/009,372, entitled “METHODS AND SYSTEMS FORESTIMATION OF PERSONAL CHARACTERISTICS FROM BIOMETRIC MEASUREMENTS,”filed Dec. 9, 2004 by Robert K. Rowe and to concurrently filed, commonlyassigned U.S. patent application Ser. No. ______, entitled“MULTISPECTRAL IMAGING BIOMETRICS,” by Robert K. Rowe (Attorney DocketNo. 020204-002810US) and U.S. patent application Ser. No. ______,entitled “MULTISPECTRAL LIVENESS DETERMINATION,” by Robert K. Rowe(Attorney Docket No. 020204-002830US), the entire disclosures of each ofwhich are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

This application relates generally to biometrics. More specifically,this application relates to multispectral imaging biometrics.

“Biometrics” refers generally to the statistical analysis ofcharacteristics of living bodies. One category of biometrics includes“biometric identification,” which commonly operates under one of twomodes to provide automatic identification of people or to verifypurported identities of people. Biometric sensing technologies measurethe physical features or behavioral characteristics of a person andcompare those features to similar prerecorded measurements to determinewhether there is a match. Physical features that are commonly used forbiometric identification include faces, irises, hand geometry, veinstructure, and fingerprint patterns, which is the most prevalent of allbiometric-identification features. Current methods for analyzingcollected fingerprints include optical, capacitive, radio-frequency,thermal, ultrasonic, and several other less common techniques.

Most of the fingerprint-collection methods rely on measuringcharacteristics of the skin at or very near the surface of a finger. Inparticular, optical fingerprint readers typically rely on the presenceor absence of a difference in the index of refraction between the sensorplaten and the finger placed on it. When the angle of light at aninterface is greater than the critical angle and an air-filled valley ofthe fingerprint is present at a particular location of the platen, totalinternal reflectance (“TIR”) occurs in the platen because of theair-platen index difference. Alternatively, if skin of the proper indexof refraction is in optical contact with the platen, the TIR at thislocation is “frustrated,” allowing light to traverse the platen-skininterface. A map of the differences in TIR across the region where thefinger is touching the platen forms the basis for a conventional opticalfingerprint reading. There are a number of optical arrangements used todetect this variation of the optical interface in both bright-field anddark-field optical arrangements. Commonly, a single quasimonochromaticbeam of light is used to perform this TIR-based measurement.

There also exists non-TIR optical fingerprint sensors. Some non-TIRcontact sensors rely on some arrangement of quasimonochromatic light toilluminate the front, sides, or back of a fingertip, causing the lightto diffuse through the skin. The fingerprint image is formed because ofthe differences in light transmission through the finger and across theskin-platen interface for the ridge and valleys. The difference inoptical transmission at the interface is due to changes in the Fresnelreflection characteristics that result from the presence or absence ofintermediate air gaps in the valleys. Some non-TIR sensors arenon-contact sensors, which use polarized light to image the surfacefeatures of the finger. In some cases the imaging system may include alinear polarizer and the illumination light may be polarized in paralleland perpendicular directions to provide two images, which are thencombined in some manner to enhance the surface features of the finger.

Although optical fingerprint readers based on TIR phenomena are one ofthe most commonly deployed types of fingerprint sensors, they aresusceptible to image-quality problems due to non-ideal conditions. Ifthe skin is overly dry, the index match with the platen will becompromised, resulting in poor image contrast. Similarly, if the fingeris very wet, the valleys may fill with water, causing an opticalcoupling to occur all across the fingerprint region and greatly reduceimage contrast. Similar effects may occur if the pressure of the fingeron the platen is too little or too great, the skin or sensor is dirty,the skin is aged and/or worn, or overly fine features are present suchas may be the case for certain ethnic groups and in very young children.These effects decrease image quality and thereby decrease the overallperformance of the fingerprint sensor. In one recent study, 16% offingerprint images were found to be of suboptimal image quality as aresult of these effects. In some cases, commercial optical fingerprintreaders incorporate a thin membrane of soft material such as silicone tohelp mitigate some of these effects and restore performance. As a softmaterial, the membrane is subject to damage, wear, and contamination,limiting the use of the sensor before it requires maintenance.

Biometric sensors, particularly fingerprint biometric sensors, aregenerally prone to being defeated by various forms of spoof samples. Inthe case of fingerprint readers, a variety of methods are known in theart for presenting readers with a fingerprint pattern of an authorizeduser that is embedded in some kind of inanimate material such as paper,gelatin, epoxy, latex, or the like. Thus, even if a fingerprint readercan be considered to reliably determine the presence or absence of amatching fingerprint pattern, it is also critical to the overall systemsecurity to ensure that the matching pattern is being acquired from agenuine, living finger, which is difficult to ascertain with manyexisting sensors.

There is accordingly a general need in the art for improved methods andsystems for biometric sensing.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide a method of performing a biometricmeasurement on an individual. A purported skin site of the individualdisposed relative to a platen is illuminated under a plurality ofdistinct optical conditions during a single illumination session. Atleast one of the plurality of distinct optical conditions includesilluminating the purported skin site with light at an angle greater thana critical angle defined by an interface of the platen with an externalenvironment. At least another of the distinct optical conditionsincludes illuminating the purported skin site with light at an angleless than the critical angle. Light scattered from the purported skinsite is received separately for each of multiple of the plurality ofdistinct optical conditions to derive a multispectral image of thepurported skin site.

In one embodiment, portions of the purported skin site are in contactwith the platen. In such an embodiment, light received after scatteringfrom the purported skin site may comprise light received afterscattering from positions where the light incident on the purported skinsite at the angle greater than the critical angle coincides withpositions where the purported skin site in contact with the interface toderive an image of surface structure of the purported skin site.

In different embodiments, the multiple distinct optical conditions mightcomprise distinct polarization conditions, distinct illumination angles,and/or might comprise distinct wavelengths of illumination light. In oneembodiment, the platen comprises a thickness of material; illuminatingthe purported skin site with light at the angle greater than thecritical angle comprises reflecting the light multiple times throughtotal internal reflectance within the thickness of material beforepassing through the interface of the platen onto the skin site. Inanother embodiment, the light received under each of the multipleoptical conditions is received by the same light detector.Alternatively, light scattered under the at least one of the pluralityof distinct optical conditions may be received with a first imager andlight scattered under the other of the distinct optical conditions maybe received with a second imager different from the first imager. Thelight scattered under the at least one of the plurality of distinctoptical conditions may be received under dark-field or bright-fieldconfigurations in different embodiments. A position of the purportedskin site may be substantially unchanged during the single illuminationsession.

Another set of embodiments provides a biometric measurement system. Thebiometric measurement system comprises a platen, an illumination source,a light detector, and a controller. The platen is adapted for placementof a purported skin site by an individual. The controller is interfacedwith the illumination source and the light detector. The controllerincludes instructions to illuminate the purported skin site under aplurality of distinct optical conditions during a single illuminationsession in which a position of the purported skin site is substantiallyunchanged. At least one of the plurality of distinct optical conditionsincludes illuminating the purported skin site with light at an anglegreater than a critical angle defined by an interface of the platen withan external environment. Another of the distinct optical conditionsincludes illuminating the purported skin site with light at an angleless than the critical angle. The controller also includes instructionsto derive a multispectral image of the purported skin site from lightreceived by the light detector after scattering from the purported skinsite for each of multiple of the plurality of distinct opticalconditions. In some instances, the biometric measurement system isconveniently comprised by a portable electronic device or may becomprised by a turnstile.

In several embodiments, the platen comprises a first thickness ofmaterial. The illumination source is disposed to illuminate thepurported skin site with light at the angle greater than the criticalangle by reflecting the light multiple times through total internalreflectance within the first thickness of material before passingthrough the interface onto the skin site. The platen may sometimescomprise a second thickness of material through which the lightscattered from beneath the surface of the purported skin site ispropagated before being incident on the purported skin site. The secondthickness of material may include a hole disposed relative to the lightdetector such that the light scattered from beneath the surface of thepurported skin site propagates through the hole to the light detector.The first and second thickness of material may be separated by athickness of the external environment. In some instances, theillumination source may comprise a first illumination source disposedwithin the first thickness of material and/or a second illuminationsource disposed within the second thickness of material.

In different embodiments, the multiple distinct optical conditions mightcomprise distinct polarization conditions, distinct illumination angles,and/or distinct wavelengths of illumination light. In one embodiment,the light detector comprises an optical axis oriented at an angle lessthan the critical angle. For example, the optical axis may be orientedsubstantially normal to the interface.

The controller may also sometimes include instructions to perform abiometric function with the derived multispectral image and derivedimage of surface structure of the skin site. For example, the biometricfunction might comprise verifying an identity of the individual.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference labels are usedthrough the several drawings to refer to similar components. In someinstances, reference labels include a numerical portion followed by alatin-letter suffix; reference to only the numerical portion ofreference labels is intended to refer collectively to all referencelabels that have that numerical portion but different latin-lettersuffices.

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 provides a front view of a multispectral biometric sensor in oneembodiment of the invention;

FIGS. 2A-2D provide four views of a multispectral biometric sensor shownin another embodiment of the invention;

FIG. 3A shows an embodiment of the invention that incorporatestotal-internal-reflectance illumination, multispectral-imagingillumination, and an optical prism;

FIG. 3B shows an embodiment of the invention that incorporates a slabplaten for total-internal-reflectance illumination and a separatemechanism for direct-imaging illumination;

FIG. 3C shows an embodiment of the invention that incorporates a slabplaten for total-internal-reflectance illumination and a separate slabfor direct-imaging illumination;

FIGS. 4A-4C illustrate effects of different polarization configurationsused in different embodiments;

FIG. 5 provides a schematic illustration of a multiple-camera embodimentof the invention;

FIG. 6 provides a flow diagram illustrating methods for performingbiometric determinations of identity and/or sample authenticity usingmeasurements at a plurality of distinct optical conditions;

FIG. 7 is a graph showing the absorbance of hemoglobin at differentlight wavelengths;

FIG. 8 provides a schematic illustration of integration of amultispectral biometric sensor with a portable electronic deviceaccording to embodiments of the invention;

FIGS. 9A and 9B compare results of total-internal-reflectance andmultispectral-imaging measurements of a fingertip under non-idealconditions (COLOR);

FIGS. 10A and 10B illustrate image characteristics with the presence ofoptically clear film;

FIG. 11A provides a top view of a tissue phantom used is simulations fordemonstrating certain aspects of the invention;

FIG. 11B provides examples of multispectral images at differentwavelengths for the skin phantom shown in FIG. 11A;

FIGS. 12A and 12B provide results of multispectral images taken forillustrative spoof samples (COLOR);

FIG. 13 provides a comparison of multispectral images taken for livingand prosthetic fingers to illustrate spoof detection (COLOR);

FIGS. 14A-14D show results of a multiperson study conducted to evaluatethe use of multispectral imaging data in providing complementaryinformation to improve fingerprint biometric performance; and

FIGS. 15A-15D illustrate an exemplary embodiment in which a sensor isintegrated with a turnstile.

DETAILED DESCRIPTION OF THE INVENTION

1. Overview

Embodiments of the invention provide methods and systems that allow forthe collection and processing of biometric measurements. These biometricmeasurements may provide strong assurance of a person's identity, aswell as of the authenticity of the biometric sample being taken, and maybe incorporated within a number of different types of devices, such ascellular telephones, personal digital assistants, laptop computers, andother portable electronic devices, as well as stand-alone devices forphysical or logical access. The common characteristic of the methods andsystems of the present invention is the application of multiple distinctoptical configurations used to collect a plurality of image data duringa single illumination session. In some instances, methods and systemsare provided for the collection and processing of data using a sensorwith two distinct imaging systems. In other instances, the methods andsystems disclosed pertain to data collected using a sensor with a singlecamera.

The sensors of the present invention provide for an information-richdataset that results in increased security and usability relative toconventional sensors. The increased security derives from combininginformation from multiple images that represent distinct opticalcharacteristics of the material being measured. These characteristicsprovide sufficient information to be able to distinguish between livinghuman skin and various artificial materials and methods that might beused to attempt to spoof the sensor. As well, increased security isderived from the aspect of the present invention that provides amechanism to perform measurements across a wide range of environmentaland physiological effects. The robust and reliable sampling means thatsystem security standards do not have to be relaxed to compensate forpoor image quality.

Enhanced sensor usability is achieved by reducing the constraints on theindividual for precise contact and positioning, as well as therequirement that the individual's skin has particular qualities. Aswell, the ability to extract subsurface biometric information fromimages collected under certain optical conditions provides a mechanismfor performing biometric determinations even in those cases where thesurface features are missing or damaged. In this way, the multispectralmeasurements made in embodiments of the present invention areadvantageously robust to non-ideal skin qualities, such as dryness,excess wetness, lack of resilience, and/or worn features such as aretypically associated with the elderly, those who perform significantmanual labor, or those whose skin is exposed to chemicals, such ashairdressers or nurses.

The set of all images collected under a plurality of distinct opticalconditions during a single illumination session is referred to herein as“multispectral data”. The different optical conditions may includedifferences in polarization conditions, differences in illuminationangle, differences in imaging angle and differences in illuminationwavelength. In some optical conditions the resulting images aresignificantly affected by the presence and distribution of TIR phenomenaat the interface between the sample and the platen. These images arereferred to herein as “TIR images.” In some optical conditions, theresulting images are substantially unaffected by the presence or absenceof TIR effects at the platen. These images are referred to herein as“direct images”.

Skin sites applicable to the multispectral measurements described hereininclude all surfaces and all joints of the fingers and thumbs, thefingernails and nail beds, the palms, the backs of the hands, the wristsand forearms, the face, the eyes, the ears, and all other externalsurfaces of the body. While the discussion below sometimes makesspecific reference to “fingers” in providing examples of specificembodiments, it should be understood that these embodiments are merelyexemplary and that other embodiments may use skin sites at other bodyparts.

In some embodiments, a sensor provides a plurality of discretewavelengths of light that penetrate the surface of the skin, and scatterwithin the skin and/or underlying tissue. As used herein, reference to“discrete wavelengths” is intended to refer to sets of wavelengths orwavelength bands that are treated as single binned units—for each binnedunit, information is extracted only from the binned unit as a whole, andnot from individual wavelength subsets of the binned unit. In somecases, the binned units may be discontinuous so that when a plurality ofdiscrete wavelengths are provided, some wavelength between any pair ofthe wavelengths or wavelength bands is not provided, but this is notrequired. In some instances, the wavelengths are within theultraviolet—visible—near-infrared wavelength range.

A portion of the light scattered by the skin and/or underlying tissueexits the skin and is used to form an image of the structure of thetissue at or below the surface of the skin. In some embodiments, such animage may include a fingerprint image, where the term “fingerprint” isused broadly herein to refer to any representation of any skin site withdermatoglyphic features.

A detailed description is provided below of examples of multispectralsystems that may accordingly be used in embodiments of the invention,but such a description is not intended to be limiting since othertechniques may be used in alternative embodiments.

2. Single-Camera Multispectral Imaging

A first example of an embodiment that used multispectral imagingcomprising a plurality of different illumination angles is shown inFIG. 1. The multispectral sensor 101 in this embodiment comprises one ormore sources of light 103 that illuminate the finger at an angle, θ₁,one or more sources of light 133 that illuminate the finger at an angle,θ₂, and an imaging system 123, which may comprise a digital imagingsystem. In a preferred embodiment, angle θ₁ is less than the criticalangle θ_(C), and angle θ₂, is greater than the critical angle. Thenumber of illumination sources may conveniently be selected to achievecertain levels of illumination, to provide for multiple illuminationwavelengths, to provide for multiple polarization conditions, to meetpackaging requirements, and to meet other structural constraints of themultispectral biometric sensor 101.

Illumination passes from the sources 103, 133 through illuminationoptics that shape the illumination to a desired form, such as in theform of flood light, light lines, light points, and the like. Theillumination optics 105, 135 are shown for convenience as consisting oflenses but may more generally include any combination of one or morelenses, one or more mirrors, and/or other optical elements. Theillumination optics 105, 135 may also comprise a scanner mechanism (notshown) to scan the illumination light in a specified one-dimensional ortwo-dimensional pattern. The light sources 103, 133 may comprise a pointsource, a line source, an area source, or may comprise a series of suchsources in different embodiments. The sources 103, 133 may be narrowband sources such as monochromatic LED's and laser diodes or may bebroad band sources such as white-light LED's or incandescent sources. Inthe case where light sources 103, 133 comprise a series of sources, theseries of sources may be of the same wavelength or differentwavelengths. The different sources 103, 133 may be configuredidentically or they may differ from each other.

After the light passes through the illumination optics 105, 135 itpasses through a platen 117 and illuminates the finger 119 or other skinsite so that reflected light is directed to an imaging system 123. Theplaten 117 may be configured in such a manner that illumination enteringthe platen will traverse the platen 117 at the desired angles. In thecase of illumination system 109, which illuminates the skin site at anangle, θ₁, the facet, 117 b, is oriented roughly normal to theillumination axis. Likewise, in the case of the illumination system 139,which illuminates the skin site at an angle, θ₂, the facet 117 c isoriented at a steeper angle to be approximately normal to thecorresponding illumination angle.

In a preferred embodiment, angle θ₁ is less than the critical angle andangle θ₂ is greater than the critical angle, which is defined as theangle at which total internal reflection occurs. Inset 151 shows thegeometry associated with calculating the critical angle at the interfacebetween two materials with different indices of refraction. As known inthe art, refraction of light will generally occur at such an interface.The angle of refraction will be different for different illuminationangles and will be governed by an equation of the form:n ₀ sin Θ₀ =n ₁ sin Θ₁,where n₀ is the refractive index in medium 0, n₁ is the refractive indexin medium 1, and the angles, θ₀ and θ₁, are measured in the respectivemedia from the normal to the interface. When n₀ is less than n₁, thecritical angle, θ_(c), is given by:${\Theta_{C} = {\sin^{- 1}( \frac{n_{0}}{n_{1}} )}},$

In the case where n₀ is approximately equal to 1.0 corresponding to airand n₁ is approximately equal to 1.5 corresponding to a type of glass,the critical angle is approximately 41.8 degrees. In a case such asthis, the illumination angle, θ₁, may range from 0 up to approximately40 degrees while illumination angle θ₂ will be at an angle greater than41.8 degrees but less than the critical angle defined by the interfacebetween the platen and the finger skin. For skin with an index ofrefraction of 1.4 this secondary critical angle is approximately 70.0degrees.

In the case where θ₁ is less than the critical angle, the illuminationlight from subsystem 109 passes through the top facet of the platen 117a and will illuminate all portions of the finger 119 if present on orabove the platen 117. A portion of the light illuminating the finger 119will be reflected from the skin surface while a second portion of thelight will pass into the skin and undergo optical effects such asscattering and absorption. Generally, a portion of the light that entersthe finger skin will scatter back out of the skin and pass back into theplaten 117.

In the case where θ₂ is greater than the critical angle and in theabsence of a finger, light from subsystem 139 will not pass throughfacet 117 a and will be reflected back into the platen 117. Light willtraverse the interface at facet 117 a only in those locations that skinor other media with a suitable index of refraction is in direct opticalcontact with the facet 117 a. At the points of contact between theplaten 117 and finger 119, light will be partially reflected by thesurface of the skin and partially absorbed by the skin in a mannerdescribed previously. However, in cases where the illuminationwavelength is such that light does not propagate very far in the skinbefore being absorbed, the light scattered at each point of contact iswell localized to that point. This is the case for a variety ofdifferent wavelengths in the ultraviolet, visible and near infraredspectral regions. In particular, visible light with a wavelength shorterthan approximately 580 nm is highly absorbed by hemoglobin and thusremains well localized to the point of illumination.

When illuminated by either subsystem 109 or 139, light scattered andreflected by the skin may be imaged with an appropriate imaging system.FIG. 1 illustrates an embodiment in which the imaging system 123comprises a digital imaging system having a digital array 115 anddetection optics 113 adapted to focus the light reflected from theobject onto the array. For example, the detection optics 113 maycomprise a lens, a mirror, a pinhole, a combination of such elements, ormay use other optical elements known to those of skill in the art. Thearray 115 may comprise a silicon imaging array, such as a CCD or CMOSarray, an InGaAs array, or other detector arrays as known in the art. Insome instances, the imaging system 123 may also comprise an opticalfilter 121. The optical filter 121 may be a short-wavelength passfilter, which substantially blocks light of wavelengths longer than theillumination wavelength range. Such a configuration has been found bythe inventors to provide advantageous performance in the presence ofbright, broad-band ambient lighting, since wavelengths of light longerthan approximately 580 nm may substantially traverse the finger. Inbright sunlight, this long wavelength light may saturate the detectorarray 121 preventing the acquisition of an image. Blocking suchlong-wavelength light with filter 121 while passing all desiredillumination wavelengths may thus be beneficial.

In some instances the filter 121 may be a color filter array, which mayfurthermore be incorporated as part of the digital array 115. The colorfilter array 121 may comprise a red-green-blue filter array in thewell-known Bayer pattern. In some instances, the filter elements mayfunction to transmit wavelengths that differ from the standardred-green-blue wavelengths, may include additional wavelengths, and/ormay be arranged in a pattern that differs from the Bayer pattern. Ininstances where such a color filter array 121 is included, theillumination source(s) may be white-light or broadband source(s).Alternatively, the illumination source(s) 103, 133 may comprise aplurality of narrowband sources, such as LEDs, with central wavelengthsthat are within the pass bands of filter elements comprised by the colorfilter array 121. In some embodiments, the illumination light isprovided within a wavelength range of approximately 400-1000 nm. Inother embodiments, wavelengths within the visible range of the spectrum,i.e. in the range of about 400-700 nm, are used. In some cases, aplurality of substantially discrete wavelengths are used, such as in anembodiment where three illumination wavelengths correspond to red,green, and blue colors at about 600, 540, and 450 nm respectively.

The sensor layout and components may advantageously be selected tominimize the direct reflection of the illumination sources 103, 133 intothe digital imaging system 123. In one embodiment, such directreflections are reduced by relatively orienting the illumination anddetection optics such that the amount of directly reflected lightdetected is minimized. For instance, optical axes of the illuminationoptics 105 and the detection optics 113 may be placed at angles suchthat a mirror placed on the platen surface 117 a does not direct anappreciable amount of illumination light into the detection subsystem123. In a similar way, the detection optics 113 should be oriented toavoid light from illumination subsystem 139 that undergoes totalinternal reflectance at platen surface 117 a.

In one embodiment, the optical axis of the imaging subsystem 123 isoriented in a manner that enables the imager to “see through” the platensurface 117 a rather than be affected by total internal reflectance atthis surface. In this way, the imaging subsystem 123 is able to obtainimages of light scattered and reflected by a finger at all points ratherthan just those points where the finger is in contact and of necessaryindex of refraction. This constraint may be generally met by orientingthe imaging subsystem 123 with an angle less than the critical angle,θ_(c). In some cases, the imaging subsystem, 123, may be orientedapproximately normal to the platen facet 117 a.

In another embodiment, the optical axis of the imaging subsystem 123 isoriented in a manner that causes the imager to only see light from thosepoints where the skin of proper index of refraction is in opticalcontact with the platen surface 117 a. This can be achieved by placingthe imager 123 at an angle greater than the critical angle, θ_(C). Ifthe imager is located at such a position and angle that it sees theillumination light in the absence of a finger or other material touchingthe surface 117 a, it is referred to as a “bright-field” imagingcondition. In such a case, points of contact with the finger will appearrelatively dark. If the imager is located at such a position and anglethat it does not see the illumination light in the absence of a fingeror other material touching the surface 117 a, it is referred to as a“dark-field” imaging condition. In such a case, points of contact withthe finger will appear relatively light. In some cases, opticalbaffling, optical black coating, and/or other techniques known in theart may be employed to reduce the effect of spuriously scattered lightand thereby increase image quality in either imaging condition, andparticularly in the dark-field imaging condition.

The specific characteristics of the optical components comprised by themultispectral sensor 101 may be implemented to meet differentform-factor constraints. For example, in an embodiment where themultispectral sensor is implemented in the top of a gear shift as partof a system to verify the identity of a driver of a vehicle, the lightsources 103, 133 and digital array 115 might not fit within thegear-shift handle as constructed. In such an embodiment, an opticalrelay system may be implemented. For example, relay optics that compriseindividual lenses similar to those in a bore scope may be used, oralternatively optical fibers such as used in orthoscopes may be used. Inother cases, the optical paths of the illumination subsystems, 109, 139,and/or the detection subsystem, 123, may be folded through the use ofmirrors to reduce the overall size. Still other techniques forimplementing an optical relay system and/or folding the optical systemswill be evident to those of skill in the art. In this way, components ofthe sensor may be located remotely from the sampling surface or beconfigured to fit other form-factor constraints.

The multispectral sensor may take multiple images in sequence during anillumination session. For example, in the case of multiple sources ofdifferent wavelengths, polarization conditions, and/or angles, the firstsource may illuminate during which time the camera acquires and storesan image. The first source is then extinguished and a second source isilluminated during which time a second image is acquired and stored.This sequence then continues for all sources and may further include a“dark” image that is collected with no sources illuminated. Also any orall of the image conditions may be repeated an arbitrary number of timesduring an illumination session. The resulting images may be combined invarious ways for subsequent processing. For example, difference imagesmay be generated between each of the illuminated states and the darkimage. The difference between these two types of images allows theeffect of illumination to be separated from background illumination. Thedifference images may then be used for further processing according toother aspects of the invention.

FIGS. 2A-2D provide an example of a structure for a multispectral sensorwith multiple illumination subsystems in which the optical conditionsfurther include differences in polarization conditions. The basicstructure of the sensor 201 is similar to that of FIG. 1, but multipleillumination systems have been depicted. Two illumination subsystems 209are placed at angles greater than the critical angle, causing totalinternal reflectance to occur at platen surface 117 a in the absence ofdirect contact with skin. Four illumination subsystems, 213, areoriented at angles less than the critical angle with respect to surface117 a. Polarizers 207, 211 have been added to the illuminationsubsystems 209, 213 and a polarizer 205 has been added to the imagingsystem 203. The polarizers 205, 207, 211 may be linear, circular,elliptical, or some combination of the these. The illumination sources103 (4) and 133 (2) may be broadband or narrowband. If narrowband, thesources may all be the same wavelength or may be substantially differentwavelengths. The polarizers 207 and 211 may also provide a “crossedpolarization” arrangement or a “parallel polarization” arrangement onsome or all of the illumination subsystems 209, 213. One or more of theillumination subsystems 209, 213 may have the polarizer omitted,producing randomly polarized illumination light.

In the case that one of the illumination subsystems 209, 213 provides acrossed polarization arrangement, the polarizer 207, 211 is disposed andoriented to provide illumination light that is polarized orthogonally tothe polarization at the imaging system 203. Such orthogonality hasutility in ensuring that detected light has undergone multiple scatterevents, such as at the skin site 119, since other light will be blocked.This characteristic of crossed polarizers is particularly pronounced inthe case where the illumination subsystem 213 is oriented at an angleless than the critical angle. In this case, in the absence of crossedpolarizers, light may be detected from surface reflections from theskin, shallow scatter events, and deep scatter events. When crossedpolarizers are used, surface and shallow-scattering phenomena aresignificantly attenuated. Conversely, parallel polarizer may beadvantageously employed to accentuate surface features and shallowscattering effects. Random polarization can also be employedadvantageously, particularly in conjunction with at least one otherpolarization state.

In the case of linear polarizers, a crossed polarization arrangement maybe provided by having the illumination polarizers 207, 211 oriented sothat their axes are separated by approximately 90° from the axis of thedetection polarizer 205. In alternative embodiments where the polarizersare circular polarizers, the orthogonality of the crossed polarizationarrangement may be achieved by having circular polarizers of oppositesense (i.e. right hand and left hand). Further, in the case of linearpolarizers, a parallel polarization arrangement may be provided byhaving the illumination polarizers 207, 211 oriented so that their axesare approximately parallel to the axis of the detection polarizer 205.In alternative embodiments where the polarizers are circular polarizers,parallel polarization may be achieved by using the same sense ofcircular polarization Due to the effect of the polarizers, multipledifferent optical conditions can be achieved by changing thepolarization state of the system, even when only a single illuminationwavelength is being used. Of course, multispectral conditions may alsocomprise the use of different illumination wavelengths, differentillumination angles, and different imaging angles, among othercombination of different optical conditions.

Further utility is derived from the observation that the crosspolarization arrangement greatly reduces the visibility of latent printsleft on the platen 117 by previous users, thus providing improved imagequality and reducing the likelihood of spoofing by “reactivating” thelatent prints. The utility of the arrangement also extends toconventional optical fingerprint readers. In particular, dark-fieldoptical fingerprint systems are well-suited for the additional ofpolarizing elements in such an arrangement.

More generally, effects such as latent prints may be identified andsegmented from the resulting multispectral data based upon their uniqueoptical characteristics. For example, the optical qualities of latentprints with respect to different polarization conditions differ fromliving human tissue. Similarly, the spectral characteristics of latentprints as a function of wavelength and illumination angle are also quitedifferent from living human tissue. An analysis of the spectralproperties of the multispectral data can thus provide a means toseparate the real tissue image from artifacts due to latent printsthrough techniques such as spectral unmixing, as known in the art.Spectral analysis may also be used to perform image segmentation,defining and isolating the region of the image that contains tissue datafrom the image background. In a similar manner, the totality ofinformation available in the multispectral dataset of the presentinvention is well suited to distinguishing between genuine human skinand various attempts to use artificial samples or other means to spoofthe sensor. The composite optical characteristics of skin over multiplewavelengths, polarization conditions and illumination angles is distinctfor human skin, and can be employed to distinguish between skin and manydifferent classes of materials that might be used in an attempt to spoofthe sensor.

The embodiments described in connection with FIGS. 3A-3C combine directimaging with a variant of TIR imaging in a single integrated unit basedon a single camera. As shown in FIG. 3A, the camera 314 may be orientedat an angle less than the critical angle Θ_(C) and, in some cases, isoriented to be normal to the finger 304 or other skin site. As such thecamera 314 “sees” all of the finger 304 whether or not portions of itare in direct optical contact with the platen 306. One or more TIR-likeimages may be collected by illuminating with a light source 310 orientedat an angle greater than the critical angle Θ_(C). As described aboveand as known in the art, light from the illuminator 310 will only passinto the skin site 304 at points at which TIR is frustrated by thedirect skin-glass contact. Furthermore, if the wavelength(s) of theillumination source 310 are chosen to be wavelengths at which the skinis highly scattering and/or highly absorbing, the light that penetratesthe skin at a particular point and is scattered back out will besubstantially detected in that same region of the skin. Light that isnot transmitted into the skin will be reflected and strike an opticallyblack surface 308 or other form of light dump. In this manner, thearrangement is able to provide one or more TIR images (generated fromone or more illumination wavelengths and/or polarization conditions) byrelying on illumination-side critical-angle phenomena.

In such a system, the same camera 314 can also be used to acquiredirect-imaging data generated by one or more illumination sources 312oriented at an angle less than the critical angle Θ_(c). The source(s)312 may also incorporate optical diffusers and/or optical polarizers(not shown).

A second mechanism of introducing TIR illumination into the skin site304 is shown in FIG. 3B. In this embodiment, a substantially planar slabof material 320 such as glass, acrylic, or other suitable material isoriented as a window. One or more illumination sources 322 such as LED'smay be mounted on a side of the planar platen 320 in such a way that asubstantial portion of the emitted light is reflected multiple timesthrough TIR reflections 328; a small portion of the emitted light is attoo steep an angle to support TIR and is transmitted 324 in closeproximate to the source 322. Light that undergoes multiple TIRreflections passes through the platen 320 at the points where there iscontact with the skin site 304 when the skin has appropriate opticalcharacteristics. This transmitted light 330 thus illuminates the skinsite 304 in a manner that allows capture of a TIR image by the camera314. In cases where it is not practical to embed the light source 322directly in the platen 320, the light source 322 may be mountedexternally on the outside of the platen 320. A simple lens or otheroptical element may be used to efficiently couple light from theexternal source to the platen 320. The multispectral imaging isperformed using the same camera 314 and with light sources 312 asdescribed in connection with FIG. 3A.

The slab illumination concept of FIG. 3B is further extended to providedirect illumination with the embodiment shown in FIG. 3C. In thisinstance, a second slab 330 is placed below the TIR slab 320. The twoslabs 330 and 320 may be separated, such as by providing an air gap inone embodiment through the use of spacers 334. The direct-imaging slab330 may incorporate features such as a hole and bevels to allow thelight to escape at points that cause broad illumination of the skin site304. Etching, scoring, diffuse coating, and/or features molded into theupper or lower surfaces of the direct-imaging slab 330 may be used tocause the light to be emitted from the slab 330 at the desiredlocation(s). One or more illumination sources 332 for the direct imagingmay be mounted on the side of, or in, the second slab 330 to provide themultispectral illumination. Collection of both TIR and direct imagingdata may be performed with a single light detector 314, similar to theembodiments described in connection with FIGS. 3A and 3B.

Merely by way of example, the inventors have constructed operatingmodels of a one-camera multispectral imaging system similar to thatdepicted in FIG. 3B using the following specific components. The digitalarray was provided with a monochrome 640×480 CCD camera, namely aLumenera model #LU-070, interfaced to a PC host through a USB interface.The illumination sources 312, 322 comprised 24 high-brightness, 5 mmdia. Packaged LEDs powered by a laboratory power supply and controlledthrough a USB solid-state relay (OnTrak ADU218). Software control of theLEDs, the imager, and the associated image processing was performed withcustom software written to operate within a MATLAB environment(Mathworks, Matlab 7.0).

The LEDs comprised four different groups of Vishay LEDs (TLCXXXX) ofnominally blue, green, yellow, and red colors. Four of each color wereinserted into each of the sides of a square acrylic platen to provideTIR illumination in a manner similar to component 322 in FIG. 3B toprovide direct illumination. In addition, 4 blue LEDs and 4 green LEDswere mounted near the camera axis in a manner similar to components 312in FIG. 3B. Polarizing film (Edmund Optics, NT38-495) was placed on thecamera lens. Additional pieces of the polarizing film were cut out andused to cover the direct illuminators in some experiments. Both paralleland perpendicular polarization configurations were investigated, as wasthe case where the direct illumination sources were left unpolarized.

The inventors used the system to investigate the characteristics andutility of different types of polarization conditions. In particular theinventors examined the cases of parallel polarization, perpendicularpolarization and random polarization. The results of the investigationcan be better understood by referring to FIG. 4A-C, which illustrate theeffect of ideal polarizers in an imaging system. FIG. 4A illustrates thecase of a linear polarizer used in the detection arm and a parallellinear polarizer used in the illumination arm. In this case, the lightdetected from a material such as skin is a combination ofsurface-reflected light (I_(S)) and light from the subsurface skin(I_(D)). In contrast, FIG. 4B illustrates the case of crossed orperpendicular polarizers where under ideal circumstances the detectedlight originates only from subsurface optical interactions (I_(D)).Finally, FIG. 4C illustrates the case wherein the polarizer is omittedfrom the illumination arm and assuming the source is randomly polarized.In this case the resulting signal comprises both surface (I_(S)) andsubsurface (I_(D)) portions, but I_(D) is twice the magnitude as wasobserved in the case of parallel polarization, FIG. 4A. From this, onecan see the manner in which parallel polarization geometries emphasizethe surface features (or de-emphasize the subsurface features) relativeto the case of random polarization, and crossed polarization emphasizesthe subsurface features. Moreover, these results demonstrate that if anytwo polarization conditions are measured (i.e., perpendicular+parallel,perpendicular+random, parallel+random), the separation of surface andsubsurface effects can be achieved through an appropriate linearcombination of the two images.

2. Two-Camera Multispectral Imaging

In a number of embodiments, the multispectral principles described abovemay be integrated with other biometric techniques to provide amultifactor biometric system. Integration with a conventional TIRimaging system is particularly suitable, as illustrated in FIG. 5.Conventional optical fingerprint readers are generally configured suchthat the imager has an optical axis that is greater than the criticalangle defined by the platen-air interface, but less than the criticalangle formed by the platen-skin interface. In this way, the points ofcontact between the skin and platen cause a distinct optical contact.The points of contact so imaged can be bright or dark relative to theregion without contact depending on the exact configuration of thefingerprint sensor. Illumination in such conventional systems can begreater or less than the critical angle, depending on whether the systemis configured as a bright-field or dark-field imager. Typically, redlight is used for illumination, which readily penetrates the skin and ishighly scattered. In such cases, the illumination that passes into theskin is nonlocal and lights up a significant amount of the skin beyondthe original illumination point.

FIG. 5 shows a conventional bright-field fingerprint sensor that hasbeen modified in accordance with an embodiment of the present invention.The conventional part of the sensor is comprises a light-source 503, aplaten 511 and an imaging system 505. The imaging system 505 maycomprise lenses, mirrors, optical filters, a digital imaging array andother such optical elements (not shown). The optical axis of imagingsystem 505 is at an angle greater than the critical angle with respectto surface 511 a. The light from the source 503 passes into the platen511 and strikes a diffuse reflective coating 513, which broadlyilluminates the platen surface 511 a. In the absence of a finger 119,light undergoes TIR at surface 511 a and a portion is collected andforms a TIR image by imaging system 505. When skin of proper index ofrefraction is in optical contact with the platen surface 511 a, thepoints of contact will form relatively dark regions on the resultingimage. Other variants of conventional optical fingerprint readers usedifferent locations of sources 503 and/or imaging systems 505, toachieve a dark-field TIR image.

A second imaging system 509 may be added to the conventional geometry asshown in FIG. 5. The second imaging system 509 looks up at the finger119 through facet 511 b. Imaging system 509 has an optical axis lessthan the critical angle with respect to facet 511 a. In someembodiments, imaging system 509 is oriented approximately normal tofacet 511 a. This imaging system may comprise lenses, mirrors, opticalfilters and a digital imaging array (not shown). In this manner, whenlight source 503 is illuminated, a TIR image may be captured by camera505 while a direct image may be captured by camera 509. The inventorshave discovered that even in cases where the TIR image is adverselyaffected by water, dirt, lack of contact, dry skin, etc, the imagecaptured by camera 509 has been relatively unaffected and generallycontains usable biometric features including the fingerprint pattern.

Imaging system 509 may further incorporate an optical polarizer (notshown), which may be a linear polarizer or elliptical (e.g. circular)polarizer. As well, other light sources 507 may be added to the system.The light sources 507 may be incandescent sources such asquartz-tungsten-halogen lamps or others commonly known in the art. Thesources 507 may be other broad-band sources such as white-light LEDs orothers known in the art. The sources may be quasi-monochromatic sourcessuch as solid-state LEDs, organic LEDs, laser diodes, other kinds oflasers and quasi-monochromatic sources known in the art. The sources 507may further comprise lenses, mirrors, optical diffusers, optical filtersand other such optical elements.

The sources 507 may be substantially the same or may provide fordifferent illumination wavelengths, angles, and polarization conditions.In the latter case, one of the sources 507 a may have an opticalpolarizer (not shown) that is oriented substantially orthogonal to thepolarizer incorporated in the imaging system 509. Such an opticalgeometry tends to emphasize features of the skin that lie below thesurface. One of the light sources 507 b may incorporate a polarizer thatis substantially parallel to the polarizer used in imaging system 509,which will tend to emphasis surface features of the skin. The lightsources 507 may be of the same wavelength or of different wavelengths(with or without polarizers). The number and arrangement of sources 507may be different for different embodiments to accommodate form-factorconstraints, illumination-level constraints, and other productrequirements.

In one embodiment, the sources 507 are oriented at an angle less thanthe critical angle with respect to facet 511 a. In a preferredembodiment, sources may be located at such an angle and position suchthat no direct reflection of the source is seen by imaging system 509 or505. Such direct reflections can also be greatly mitigated through theuse of crossed-polarizer configurations, but some image artifacts willstill be generally present if the sources are in the field of view.Moreover, parallel-polarized and non-polarized configurations are verysusceptible to such back reflections.

Merely by way of example, the inventors have constructed operatingmodels of a two-camera multispectral imaging system using the followingspecific components. A Cross Match V300LC CIR fingerprint sensor wasmodified to accommodate a second imager and additional opticalcomponents The second imagen was provided with a monochrome 640×480 CCDcamera, namely a Lumenera model #LU-070, interfaced to a PC host througha USB interface. The illumination sources 103 comprised 72 discrete 0402LED packages mounted to a custom printed circuit board controlledthrough available general-purpose input/output (“GPIO”) pins availableon the Lumenera camera board. The LEDs comprised six different groups ofKingbright, APHHS005XXX LEDs at nominal wavelength values of 400, 445,500, 574, 610, and 660 nm. They were contained in two custom aluminumhousings oriented on either side of the camera lens. The housings wereused to limit scattered light and to provide a mechanism for coveringthe LEDs with a diffuser and linear polarizer. The diffusing film wasNitto Denko, H60 and the polarizers were cut from Edmund Optics,NT38-495. An additional piece of the polarizing film was cut out andplaced above the camera lens. In some instances, the source and camerapolarizers were set to be substantially orthogonal. In other instancesthe polarizers were set to be substantially parallel. In still otherinstances the source and/or camera polarizers were omitted. Softwarecontrol of the LEDs, the imager, and the associated image processing wasperformed with custom software written to operate within a MATLABenvironment (Mathworks, Matlab 7.0).

A second operating model was constructed in a similar fashion, but wascapable of simultaneous collection of different illumination wavelengthconditions. The second imaging sensor that was added to a modified CrossMatch sensor was a color imager (OmniVision OV9121) that comprised acolor filter array with red, green, and blue filters in a Bayer pattern.The illumination LEDs were chosen to have wavelengths that werecontained within each of the three filter passbands. The acquisition ofthe three different wavelengths could be performed simultaneously byilluminating all LEDs at the same time and acquiring a single image. Theraw image (not color corrected) was then sub-sampled in a mannerconsistent with the Bayer pattern to produce three sub-imagesrepresenting the red, green, and blue illumination conditions.Alternatively, a white-light source such as a white-light LED or anincandescent source could have been used. In such a case, the filters onthe color filter array would effectively select a particular set ofdiscrete wavelengths from the continuum of illumination wavelengths.

3. General System Description

Ways in which data may be collected with the structures shown in FIGS.1-3 and FIG. 5 and used for various biometric tasks are summarized withthe flow diagram of FIG. 6. While the flow diagram uses a particularordering, this ordering is not intended to be limiting. In otherembodiments, the order in which certain functions are performed may bevaried without exceeding the scope of the invention. Furthermore, theidentification of specific functions in the flow diagram is not intendedto be restrictive. In other embodiments, some of the functions may beomitted or additional functions may be performed without exceeding thescope of the invention. The method described in connection with FIG. 6may be performed using any of the structures described in connectionwith FIGS. 1-3 and FIG. 5 and with other structures that will be evidentto those of skill in the art after reading this description.

Blocks 602-606 correspond generally to the collection of an image undera first set of optical conditions, with light being generated with alight source at block 602 and directed to the platen surface at block604 to provide illumination to the skin site in cases where the fingeris present and other necessary optical conditions are met. As indicatedat block 606, the image is then collected with light from the skin siteat a light detector.

Blocks 608-612 correspond generally to the collection of a second imagecollected under a second set of optical conditions, otherwisereplicating the steps in blocks 602-606. In one embodiment, the firstand second optical conditions may comprise using two cameras with twodifferent angular orientations. In particular the first image 606 may becollected using a camera oriented at an angle less than the criticalangle with respect to the platen surface, while the camera used tocollect the second image 612 may be oriented at an angle greater thanthe critical angle. In one embodiment, the first and second opticalconditions may be conveniently measured using the same camera but differin the characteristics of the illumination light 602, 608. For example,the light generated in the first optical condition 602 may strike thesample surface of the platen at an angle less than the critical anglewhile the light generated in the second optical condition 608 may strikethe sample surface of the platen at an angle greater than the criticalangle. As another example, the light generated in the first opticalcondition 602 may be polarized at an angle that is perpendicular to thepolarizer used to collect the first image 606 while the light generatedin the second optical condition 608 may be randomly polarized. As yetanother example, the light generated in the first optical condition 602may be a particular wavelength while the light generated in the secondoptical condition 608 may be of a different wavelength.

In some embodiments, the collection of the images under the first andsecond illumination conditions may occur substantially simultaneously.For example, in the case that the two different illumination wavelengthsare used, a color filter array may be applied to the imager to allowacquisition of both wavelengths during a single acquisition interval. Ina similar way, an array that comprises different polarization elements(e.g. parallel, perpendicular, and/or none) may be used to collectimages under different polarization conditions. Such simultaneouscollection of multiple illumination conditions may be usefully employedin a “swipe” configuration. In such a configuration, a skin site ispassed over a sensor with a rectangular or “one-dimensional” aspectratio and a series of slit images are collected. These separate imagesmay then be recombined on “stitched” back together to form a singlecomposite two-dimensional image. Such recombination techniques may beapplied to a multispectral sensor in which the different opticalconditions are collected simultaneously or sufficiently close in timerelative to the finger swipe speed.

The collected data are then used to perform a biometric function, suchas an identification, identity verification, spoof detection, producinga composite fingerprint, or estimating a personal characteristicalthough other biometric functions may be performed in otherembodiments. As indicated at block 614, multispectral analysis isperformed on the data collected at blocks 606 and 612 to identifymultispectral properties of the sample being imaged to compare it toanticipated properties of living human skin or, in some cases, tocompare it to the anticipated multispectral properties of a specificperson. For example, any of several types of discriminant techniques maybe used to perform spectral comparisons (whereby spectral information isextracted from multispectral data by ignoring the spatial informationwhile preserving the relationship of the optical properties observedacross the different optical conditions), a number of which aredescribed in detail in commonly assigned U.S. Pat. No. 6,560,352,entitled “APPARATUS AND METHOD OF BIOMETRIC IDENTIFICATION ORVERIFICATION OF INDIVIDUALS USING OPTICAL SPECTROSCOPY,” the entiredisclosure of which is incorporated herein by reference for allpurposes. For instance, suitable discriminant techniques may includetechniques based on Mahalanobis distances, spectral residual magnitudes,K-nearest-neighbor models, and other linear or nonlinear discriminanttechniques. Multispectral imaging techniques as described herein mayprovide information on external friction ridge patterns of the skinsite, internal friction ridge patterns, composition and position ofother subsurface structures, spectral qualities of the skin site, thesize and shape of the skin site, the texture of the skin site, and otherfeatures and statistical qualities that are distinct between human skinand various artificial materials and spoofing methods.

In block 616, a composite fingerprint image may be extracted from themultispectral data using techniques described herein. In one instance aTIR image might be enhanced using the extracted fingerprint imagegenerated from one or more direct images in order to improve the overallfingerprint image quality. In another instance, there may be a linear ornonlinear numerical relationship established on parts of the image whereboth the multispectral image data and the TIR data are well defined.These parts may then be used to establish a mathematical model such aswith Principal Component Regression, Partial Least Squares, NeuralNetworks, or other methods known to those of skill in the art. The partsof the TIR image that are missing because of poor contact with theplaten or other reasons can then be estimated from the model soestablished. In another embodiment, the two entire image sets may beused, but the numerical model built using robust statistics in which therelationship is relatively unaffected by missing or degraded portions ofthe TIR image. Alternatively, numerical models may be establishedthrough the examination of previously collected multispectral image setsand then applied to new data. Furthermore, while in many instancescomparisons are performed over the images as a whole, in other instancesmore local characteristics may be used by confining the comparison to adefined portion of the images. In any case, the resulting compositeimage of purportedly improved quality may then be reported to a hostsystem for further biometric processing. Such a compositing process toproduce a single fingerprint image may enable the sensor to producebetter defined fingerprints from multiple distinct images while stillbeing compatible with systems designed to operate with conventionalsingle-image data.

In block 618, the multispectral data may be processed to produce abiometric template. In one example, a set of fingerprint images or asingle composite fingerprint image may be extracted from themultispectral data in the manner discussed. A template may then begenerated for each fingerprint image by recording fingerprint minutiapoints or other methods known in the art. In the case that a template isextracted from each of a plurality of images collected under differentoptical conditions, the templates may be simply appended together, orthey may be combined in such a way as to select those features that arecommon to multiple images while discriminating against those thatspuriously appear in a small number of images (perhaps 1). Properties ofthe multispectral data other than the fingerprint data may be extractedto form a template that may be different from the fingerprint templateor combined with the fingerprint template. Other such properties includelocations of deeper blood vessels, salient surface and subsurface lineson the skin that arise from scarring and/or skin aging, skin texture foreach optical condition and across optical conditions, overall spectralcharacteristics, overall finger size and shape.

In block 620, the multispectral data may be processed to produceestimates of various personal characteristics using various methods suchas those described in commonly assigned U.S. patent application Ser. No.11/009,372, entitled “METHODS AND SYSTEMS FOR ESTIMATION OF PERSONALCHARACTERISTICS FROM BIOMETRIC MEASUREMENTS,” filed Dec. 9, 2004 byRobert K. Rowe, the entire disclosure of which has been incorporatedherein by reference for all purposes. For example a neural network orother linear or nonlinear algorithm may be applied to the multispectraldata to estimate age or gender of the person whose finger was measured.One form of neural network well suited to such estimation tasks is aKohonen self-organizing map (SOM), which may be applied in anunsupervised mode or in a supervised mode, including learning vectorquantization (LVQ) using techniques known to one familiar in the art.

Block 622 describes the archival of the raw multispectral data for laterretrieval and processing. The data may be archived in the original formor compressed using either a lossy or lossless compression algorithm.The multispectral data may also be preprocessed in some manner and theresults of the preprocessing stored with or instead of the originaldata.

4. Spoof Detection

It has been noted above that embodiments of the invention have superiorspoof-detection capabilities. In many instances, these capabilitiesderive from the sensitivity of the multispectral imaging tophysiological features that are indicative of a living state of theobject being imaged. In particular, it is possible to determine a“liveness” state of an object by ensuring spectral consistency withliving material. While a system that identifies only fingerprints mightbe fooled by a simulated finger having a duplicate of an authorizedfingerprint pattern, embodiments of the invention that use multispectralimage data are capable of discriminating between such simulatedstructures and actual living structures.

In particular, skin is a complex organ made up of multiple layers,various mixtures of chemicals, and distinct structures such as hairfollicles, sweat glands, and capillary beds. The outermost layer ofskin, the epidermis, is supported by the underlying dermis andhypodermis. The epidermis itself may have five identified sublayers thatinclude the stratum corneum, the stratum lucidum, the stratumgranulosum, the stratum spinosum, and the stratum germinativum. Thus,for example, the skin below the top-most stratum corneum has somecharacteristics that relate to the surface topography, as well as somecharacteristics that change with depth into the skin. While the bloodsupply to skin exists in the dermal layer, the dermis has protrusionsinto the epidermis known as “dermal papillae,” which bring the bloodsupply close to the surface via capillaries. In the volar surfaces ofthe fingers, this capillary structure follows the structure of thefriction ridges on the surface. In other locations on the body, thestructure of the capillary bed may be less ordered, but is stillcharacteristic of the particular location and person. As well, thetopography of the interface between the different layers of skin isquite complex and characteristic of the skin location and the person.

Blood as distributed in the living tissue has distinct and strongabsorbance bands in the visible region of light, as illustrated with thegraph in FIG. 7. In this graph, the absorbance of hemoglobin at bloodconcentrations is shown for deoxygenated blood along curve 702 and foroxygenated blood along curve 704. Fortuitously, the visible wavelengthsspan an interesting region of the optical spectrum of blood, with therelationship of the red, green, and blue light being representative ofspectrally active features in skin. A multispectral sensor may be usedto make a static spectral reading of a sample, either when it touchesthe sensor surface or at a remote distance. In addition, thedirect-imaging component of the sensor may illuminate the skin site asit moves to touch the sensor surface as well as during the time periodimmediately following the first contact between finger and sensor.

Information may be drawn from either or both of the static and dynamicphases to ensure that spectral qualities match those of living tissue.For example, during the dynamic phase, a change in blood distributionand/or color of the skin may be observed in the vicinity of the sensoras the skin blanches in response to pressure being applied to the skinsite. In addition, areas of the skin may show a distinct pooling ofblood, especially those regions at the perimeter of the area of contactbetween the finger and sensor. This blanching and/or pooling of bloodprovides an identifiable set of changes to the corresponding images. Inparticular, wavelengths less than approximately 580 nm, which are highlyabsorbed by the blood, are seen to get brighter in the region ofblanching and darker in areas of blood pooling. Wavelengths longer thanapproximately 580 nm are seen to change much less during blanchingand/or pooling. The presence, magnitude, and/or relative amounts ofspectral changes that occur while the skin site touches the sensor canthus be used as an additional mechanism for discriminating betweengenuine measurements and attempts to spoof the sensor.

In embodiments where the sensor comprises both a direct imagingcomponent and an TIR component, the pattern detected by the directcomponent using one or more illumination wavelengths and/or polarizationconditions may be compared with the fingerprint pattern detected by theTIR component. In this way, internal fingerprint data due to blood andother subsurface structures is used to confirm the image of the externalfingerprint that the TIR component collects. If there is a discrepancybetween the two patterns, an attempt to spoof the sensor may beindicated and appropriate action taken. This method is especiallysensitive to such spoof attempts as placing a thin, transparent film ona fingertip that has a different fingerprint pattern—while living tissueis still presented to the sensor, the difference in fingerprint patternsdetected by the sensor nevertheless indicates a probable spoof attempt.

Other factors that can be monitored to discriminate between genuinetissue and attempts to spoof the sensor using an artificial or alteredsample of some kind. For example, the difference in images taken underdifferent polarization conditions will have certain characteristicproperties for skin that will not be the same for some other types ofmaterials. As another example, an image taken with one or morewavelengths may be monitored over time. During the specified timeinterval, changes such as those due to a pulse can be measured and usedto confirm the genuineness of the tissue. As well, changes in the imagethat result from sweating at the ridge pores may be observed and usedfor spoof detection.

5. Composite Fingerprint Generation and Biometric Template Generation

Generally, there are one or more optical conditions that will produce awell-defined fingerprint image under most environmental andphysiological conditions (assuming the finger being measured haswell-defined fingerprint features). For example, in the case where thefinger skin is moist and in good contact with the platen causingvariable TIR effects across the platen, optical conditions that produceTIR imaging (i.e. optical conditions with an illumination angle greaterthan the critical angle and/or an imaging angle greater than thecritical angle) would be expected to produce high-quality, high-contrastimages. On the other hand, if the skin is particularly dry and/or notwell coupled to the platen, direct imaging modes (i.e. illumination andimaging angles less than the critical angle) tend to produce higherquality fingerprint images. In addition the inventors have observed thatdifferent wavelengths will often define certain features (fingerprintpatterns, scars) better than other wavelengths in certain portions ofthe image field and/or for certain fingers. Also, wavelengths less thanapproximately 580 nm tend to produce features that are “blotchy” due tobeing sensitive to blood distributions in the finger skin. However, thesensitivity to blood can produce good quality fingerprint patterns incertain cases. Conversely, wavelengths longer than approximately 580 nmtend to produce more homogeneous representations of the fingerprint andother features of the finger. The inventors have also observed thatcertain polarization conditions provide good fingerprint features onlyunder certain conditions. For example, random polarization or parallelpolarization configurations tend to show well defined surface featuresin those cases where the finger is not in good contact with the platen.Generally, however, the features produced by these polarizationconfigurations are less well defined when there is good optical couplingbetween the finger and the platen. The cross-polarized configurationappears to be much less sensitive to such coupling variations.

Therefore, in order to produce a useable fingerprint biometric over awide variety of conditions, the inventors find it useful to combine thefingerprint information from the plurality of optical conditions of thepresent invention in some way. For example, each of the images for asingle illumination session taken under distinct optical conditions maybe processed separately to enhance fingerprint patterns using bandpassfilters, median filters, Fourier filtering, Gabor filters, histogramequalization, and other linear and nonlinear enhancement techniquesknown in the art. The resulting enhanced images may then be combined insome way such as taking a simple average of the images. Anothertechnique is to assess the fingerprint quality of each of the imagesusing standard-deviation metrics and other methods known in the art, andjust average together those images with sufficient quality, or toperform a weighted average across the images where the weighting isproportional to quality, etc.

Another method that has been successfully used by the inventors is topass each of the processed multispectral images separately into aminutia detection algorithm, which typically performs another set ofoperations including filtering and binarization of the images as part ofthe process to detect minutiae. These binary images may then be combinedin some fashion such as averaging or producing a median binarized image.Alternatively, each of the images can be separately processed to produceminutiae and then the minutiae may be combined in some fashion. Forexample, all minutiae may simply be compiled together or some selectionprocess may be applied to choose only those minutiae that are present onmore than a certain number of image planes. Nearby minutiae acrossmultiple image planes may be consolidated into a single minutia point.

In cases where the objective is to perform biometric matching ratherthan produce an image or a representation of an image such as atemplate, the templates produced from each of the image planes may beseparately matched to the corresponding templates from another session.The resulting match values may then be combined in some way to produce afinal match value on which a match/no-match decision may be made. Forexample, all of the individual match values may be averaged together, oronly those match values generated from high-quality images may beaveraged, or the median match value is used, or the highest or lowest isused, or a variety of other similar mathematical and logical operationsmay be applied.

The inventors have also found that it is possible to match multispectralimages to images taken with other imaging technology. For example, eachplane of a multispectral data set may be separately matched to anexisting fingerprint image. The resulting collection of match values maythen be combined in some manner (such as by calculating a mean, median,greatest value, or the like) to produce a single matching score that maythen be used to perform a matching decision. In this manner, themultispectral data may be able to be used in conjunction with existingdatabases of non-multispectral fingerprint images taken with otheroptical sensors, capacitive sensors, thermal sensors, etc.

5. Applications

In many embodiments, the multispectral sensor is incorporated into aportable electronic device. This is illustrated for one embodiment inFIG. 8, in which the portable electronic device is denoted 810. Theportable electronic device 810 may be a cellular telephone, personaldigital assistant, digital camera, laptop computer, or other portableelectronic device in different embodiments. A light detector 808 such asdescribed above in the form of a digital imaging array may beincorporated into the device 810 so that when the detector 808 is notbeing used to collect multispectral data, it may be used forconventional imaging of objects, places, and/or people, as is commonpractice. Illumination of tissue may be provided with a light sourceconfigured as an LED illumination ring 804 or may be provided as anothertype of illumination source, and imaging optics 806 are used asdescribed in connection with FIG. 1. In certain embodiments,multispectral configurations similar to those shown in FIG. 3A, 3B, or3C may be incorporated into the device 810. In some of theseembodiments, the configurations may be implemented in a swipe orstrip-imaging configuration as described earlier. In other embodiments,the configurations may include a touch or area-imaging configuration.

Although the drawing shows the skin site 802 in contact with the sensor,other equivalent embodiments may be implemented in which the skin siteis not in contact with the sensor. In some instances, the skin site maybe located a remote distance from the sensor and the optical system ofthe sensor used to image the skin site at the appropriate distance.

In some embodiments, the sensor incorporated in a portable electronicdevice may contain an optical system that enables adjustable focus. Themechanism for adjusting the focus may include one or more lenses thatmay be moved into various positions. The focusing mechanism itself maybe a conventional zoom arrangement. Alternatively, the mechanism forfocusing may use a liquid lens based on the known phenomenon ofelectro-wetting. The focusing mechanism may comprise a MEMSmicro-optical system or other similar focusing method. The focusingmechanism may comprise use of a wavefront coding method in which a phasedistortion is introduced that facilitates a post-acquisition imagecorrection for different focal positions.

In a system configuration in which the portable electronic device hasbeen designed to accommodate a “close-up” or macro image of the skinsite for biometric sensing, the same optical system may be used to readan optical code such as a barcode. Such a barcode reading could, forexample, initiate a service in which product information for a productcorresponding to the UPC barcode is downloaded to the portable device toprovide the consumer with comparative pricing and performance data.Similar barcode scans may be used in other embodiments for promotionalgames or various gaming activities. The conjunction of a barcode scantaken in close temporal proximity to a biometric scan could provide anaudit trail for legal matters, including financial documents andtransactions, forensic chain-of-evidence scenarios, and a variety oflogical and/or physical security applications.

An imaging system on a portable electronic device that is configured tocollect multispectral biometric data may also be used to scan in text,graphics, or other printed matter. In the case of text, the scanned datamay be converted to an interpretable form using knownoptical-character-recognition (“OCR”) techniques. Such text recognitionmay then be used to provide input of text-translation services, copyingservices, and other such services that may be aided by a rapid andconvenient character input.

An imaging system on a portable electronic device may also be used as anoptical input device to provide a mechanism for securely inputting datainto the device for functions such as reprogramming, security overrides,and secure digital communications. The illumination components of theimaging system may be used as optical output devices in the reversedirection from the detector elements. The use of multiple, filteredwavelengths can provide for multiple high-bandwidth channels for rapidand/or robust optical communication.

The multispectral sensor may also be used as a smart switch to turn onor enable an associated device, system, or service. In such a capacity,the multispectral sensor may be set to a video-streaming mode to collectseveral frames per second. Each frame may then be analyzed to detectmotion and, if motion is detected, perform image-processing steps toconfirm that the motion is due to a finger by analyzing the overallshape, the texture, and/or the spectral qualities relative to a livingfinger. When a finger has been confirmed, the smart switch functioncould, for example, turn a device on or off. If the motion could not beconfirmed as coming from a finger, the sensor may simply resumemonitoring.

The multispectral sensor may be used as a pointing device with similarfunctionality as a touchpad commonly used on a laptop PC. Themultispectral sensor can be used in this fashion by monitoring themotion of the finger over the sensing area. Sliding the finger in alinear motion to the left can indicate a leftward motion to the PC (orcell phone, PDA, or other device), with similar effects for motions tothe right, up, down, diagonal, or other directions. The cursor of the PC(or cell phone, PDA, or other device) may then be made to move in theindicated direction, or other appropriate action may be taken. In asimilar fashion, the surface of the sensor may be tapped in differentregions to simulate a click or double-click of a conventional PC mouse.Other motions, such as circles, X's, and the like, may be used toindicate other specific actions. In the case of touching or tapping thesensor, the degree of pressure may be estimated by evaluating the degreeof blanching occurring in the skin site. In this manner, differentactions may be taken in response to a soft pressure being sensedrelative to a hard pressure.

The spectral qualities of the skin site in motion may be assessed toensure that the detected motion is from that of a skin site rather thansome spurious object. In this way, false motions can be avoided.

The sensor surface may also be used as a simple text entry device. In asimilar fashion as in the case of a pointing device, the user may makemotions with the fingertip that describe single letters or numbers,which are then accumulated by the portable electronic device.

A particular motion of the finger may be used to increase the securityof the sensing system. In such a configuration, the spectral and spatialqualities of the finger are confirmed to match those that are on recordwhile the particular finger motion that is made is assessed to ensure itis similar to the motion on record. In this way, both the fingerqualities and the motion need to match in order to determine an overallmatch.

The multispectral sensor may be used to measure the ambient lightcondition. In order to do so, an image is taken without any illuminationlight turned on at a time when a skin site is not covering the sensorsurface. The amount of ambient light may be determined from the image.Further details about ambient lighting may be derived in the case wherethe imager uses a color filter array or a similar mechanism to assessspectral characteristics of the light. The measured levels of ambientlight may then be used by the associated device to set levels fordisplay brightness, backlighting, etc. Such settings are particularlyuseful in ensuring the usability of portable electronic devices whileconserving battery usage. The same ambient light information can also beused by the multispectral sensor to dynamically set gain, offset andother imaging parameters to best accommodate the current ambientlighting environment.

6. Tests

A number of different experimental and simulation tests have beenperformed to illustrate the effectiveness of various aspects of theinvention, including the ability to provide repeatable and reliableidentity verifications and determinations. As an initial matter, theimprovement in information collection by multispectral imaging overconventional TIR imaging is illustrated for different non-idealconditions in FIGS. 9A and 9B. For example, FIG. 9A provides acomparison of a conventional TIR image in panel 602 taken from afingertip having extremely dry skin. It is plainly evident that manyfeatures are lost from the image, making identity determinations andverifications more likely to be less reliable. In contrast, the directimage provided in panel 604 shows that features that are lost with aconventional TIR image may still be collected with the multispectralimaging techniques described herein. In this example, the direct imageis generated using illumination at five different wavelengths, 475 nm,500 nm, 560 nm, 576 nm, and 625 nm, with crossed linear polarization.The panels at the right respectively show a mapping of the first threeprincipal components to red (panel 906), green (panel 908), and blue(panel 910).

FIG. 9B shows the difference in results collected as a result of thepresence of barrier materials between the skin site and the sensor forconventional TIR and for direct images. Panel 922 shows that thepresence of an air gap when taking a conventional TIR image causes noinformation at all to be received because there is no frustration of theTIR. As seen in panel 920 for the same physical arrangement, thepresence of an air gap does not significantly impair the collection ofdirect data. Conversely, the presence of pooled water causes aconventional TIR image to be swamped because the TIR is frustratedeverywhere, resulting in a black image in panel 926 that conveys nouseful information. This may also be contrasted with the direct imageshown in panel 624 under the same physical circumstances, with nosignificant impairment of the collection of biometric information.

FIG. 10 shows the effect of placing a thin, optically clear film on thefinger, which in this experiment is simply transparent tape. FIG. 10Ashows the resulting TIR image, which shows a blank region where the tapeis. In contrast, FIG. 10B shows the average direct image that wasobserved in the same set of multispectral measurements. In this case thedirect imaging used a cross-polarized configuration taken acrossmultiple illumination wavelengths. As obvious from the figure, theresulting direct images are able to see the skin structure and otherfeatures that lie below a thin film.

The ability for multispectral imaging to discern differentcharacteristics in tissue has also been tested with optical simulations.A nonsequential optical raytracing package (TracePro, version 3.2) wasused in this way to perform initial tests of the characteristics ofmultispectral imaging as used in embodiments of the invention. For thisillustration, the main components of the sensor included a light source,a glass platen, and an image plane arranged in a direct-imagingconfiguration. The illumination light was linearly polarized, uniformillumination with a specified wavelength. The platen was modeled as1-mm-thick BK7 glass. The image plane was below the platen and viewedthe sample through a crossed linear polarizer.

A 10×10 cm² tissue phantom was constructed of a 210-μm epidermis and a4-mm dermis, which was chosen to make the dermis optically infinite inextent for the simulation. The phantom incorporated various structuresfilled with blood. In particular, a series of 100×100 μm² vessels wereplaced at a spacing of 500 μm and a depth of 150 μm to simulate thecapillary structure below the friction ridges. Also, several deeperblood-filled regions were created and positioned in the dermis to assessthe ability of the system to detect and image such structures. Theoptical parameters for epidermis, dermis, and blood were taken from V.Tuchin, Tissue Optics (SPIE 2000), Chap. 1, the entire disclosure ofwhich is incorporated herein by reference for all purposes.

In addition to the components of tissue, various characteristics of theoptical interface with the sensor were also tested. One quadrant of thetissue cube included an external topography that followed the lines ofthe underlying capillaries, to simulate a well-defined set of frictionridges in contact with the sensor. The three remaining quadrants of thetissue cube were flat, with no external features, simulating a worn ormissing fingerprint pattern. Finally, one half of one of the flatquadrants had a 25-μm air gap between the sensor and the externalsurface of the tissue cube to quantify the effect of no optical contactbetween the skin and sensor.

A top view of the tissue phantom is shown in FIG. 11A. The verticallines across the entire cube correspond to the capillaries. The denserarray of lines in the lower left quadrant correspond to the portion ofthe phantom that has external ridge structure. The horizontal lines inthe upper left quadrant as well as the circular structure in the lowerquadrant demarks the segment of the quadrant in contact with the platen(upper triangle) from that with an air gap (lower triangle).

A variety of optical simulations were run over various illuminationwavelengths in the visible and very near infrared spectral regions.Optical simulations were performed by tracing 4 million rays perwavelength distributed uniformly over a 6 mm×6 mm region centered on thetissue phantom. In general, the optical simulations produced images thatclearly showed the internal structures of the skin phantom. An exampleof such a set of simulated images is shown in FIG. 11B for wavelengthsthat vary from 400 nm to 800 nm in 50-nm increments. The type, depth,and resolution of the structures in each of the images clearly dependson the illumination wavelength. In addition, it is notable that thepresence of the internal structures in the images are relativelyunaffected by the presence or absence of external skin features, as wellas whether the skin was in optical contact with the platen or not.

Spoof detection has been tested by the inventors with a number ofdifferent imitations and illustrate the ability of the multispectralsensing configurations to discriminate between living tissue and anonliving imitation to perform a liveness assessment. Some of theresults of such spoof-detection tests are presented in FIGS. 12A-13;some additional results are presented in U.S. Prov. Pat. Appl. No.60/610,802, entitled “FINGERPRINT SPOOF DETECTION USING MULTISPECTRALIMAGING,” which has been incorporated herein by reference. FIG. 12Aprovides for a static spectral test comparing measurements of an actualfingertip with measurements performed on a playdough imitationconstructed by the inventors. Panel 1202 provides a filtered pseudocolorimage of the actual fingertip, which may be compared with thecorresponding image of the playdough imitation in panel 1204.

The results of multispectral imaging comparisons are shown in panels1206-1212. Panels 1206 and 1208 respectively show the results ofMahalanobis distance and spectral residual calculations for the actualfingertip. These may be compared with panels 1210 and 1212, which showcorresponding results for the playdough imitation. It is evident fromthe results that the playdough imitation lacks multispectralcharacteristics present in the results for the actual fingertip,providing a basis for a liveness discrimination.

More impressive results are provided in FIG. 12B, which shows a similararray of panels for tests performed with a living fingertip and with anultrarealistic prosthetic fingertip whose construction was commissionedby the inventors. The prosthetic fingertip was made of a multilayersilicone structure, cast on a real and available finger, and colored tomatch the coloring of the real finger. Fine detail was included on theprosthetic, including the fine detail of fingerprints. It is apparentfrom the filtered pseudo-color images of the actual finger andprosthetic shown respectively in panels 1222 and 1224 that it isdifficult to discern which is the spoof imitation. This determinationis, however, notably in contrast with the multispectral results shown inpanels 1226-1232, with panels 1226 and 1228 respectively showing resultsof Mahalanobis distance and spectral residual calculations for theactual finger, and panels 1230 and 1232 respectively showingcorresponding results for the prosthetic. Characteristic circumferentialoptical structure is evident in the Mahalanobis distance results thatindicates lack of a liveness state for the prosthetic in panel 1230;this determination is even more pronounced with the spectral residualresults shown in panel 1232 to be similar to those for the playdoughimitation in panel 1212 of FIG. 12A.

A chromatic texture comparison is shown in FIG. 13 for the real fingerand prosthetic. In each instance chromatic texture was analyzed byseparating blue, green, and red contributions at 475 nm, 540 nm, and 625nm. Color panes are shown in the left portion of the drawing and a powerspectrum ratio for each chromatic contribution is shown in the rightportion of the drawing. For the actual finger, a finger:finger powerspectrum ratio is shown for the blue contribution 1302, the greencontribution 1304, and the red contribution 1306. Similarly, for theprosthetic, a prosthetic:finger power spectrum ratio is shown for theblue contribution 1312, the green contribution 1314, and the redcontribution 1316. These results demonstrate that significantly morepower is provided in the prosthetic spatial frequencies than in theactual fingertip, particularly for blue illumination at higher spatialfrequencies. Such a distinction enables the liveness-state determinationto be made, and therefore for a spoof detection to be made.

A multiperson study was also conducted by the inventors, with study databeing collected over a period of approximately sixteen days on fifteenadults using a two-camera system similar to FIG. 5. The nine male andsix female participants ranged in age from 20 to 50 years (mean=37.4years). All were office workers with no health issues or other notableattributes. The data were collected in New Mexico during the winterperiod in which the atmospheric conditions were relatively dry. A totalof 602 multispectral datasets were collected, with three or four imagesbeing collected from participants during each visit on each of fourfingers (left index, left middle, right index, and right middle).

Direct images were preprocessed by blurring each image wavelength planewith a Gaussian filter of sufficient width to remove the fingerprintfeatures. This smooth image was then used as the divisor for ratioing tothe original image on a pixel-by-pixel basis, a process that actedprimarily to remove effects of nonuniform illumination across theplaten. The resulting image for each plane was then bandpass-filteredusing a linear filter with a normalized passband of 0.15-0.35, whichcorresponds to real spatial frequencies of about 1.5-3.4 cycles/mm. Thisbandpass operation suppressed out-of-band variation, especiallypixel-to-pixel noise) while passing frequencies useful for fingerprintfeatures. An adaptive histogram equalization was performed to equalizethe fingerprint contrast over the image plane.

The analysis of biometric performance was produced by performing aone-to-one match between pairs of fingerprint measurements. Each of the602 sets of images was matched to all of the other images collected onthe same finger (“genuine matches”). The same number of randomlyselected nonmatching images was also compared to each of the chosenimages (“imposters”). This resulted in a total of 6,056 comparisonsbeing made for both genuine matches and imposter matches. In the case ofgenuine matches, no provision was made for eliminating pairs of imagesthat differed only in the matching order so that the effective number ofindependent matches was 3,028.

Biometric processing of the TIR data was straightforward since itprovided only a single image per measurement. The direct data, after thepreprocessing described above, were processed to extract minutiae fromeach of six image planes corresponding to different illuminationconditions (cross-polarized 445, 500, 574, 610, and 660 nm; andnonpolarized 640 μm) and matched against the corresponding direct imageplane of a second measurement. The match values from each of the siximage planes were averaged to provide a final direct image score.

FIG. 14A shows the receiver operating characteristic (“ROC”) curves forthe TRI data and for the combined direct (“MSI”) data. The ROC curveshows the relationship between the false nonmatch rate (“FNMR”) to thefalse match rate (“FMR”) in percentage units. The equal-error rate(“EER”), which is the point on the curve where the FNMR equals the FMR,is seen to be quite elevated for the TIR data (˜20%) while the EER forthe combined direct data is much lower (˜0.7%). Based on 3,028independent comparisons and binomial statistics, the 95% confidenceintervals for these two performance metrics are [18.8%, 21.6%] and[0.4%, 1.0%] respectively. The differences of these performanceestimates are therefore statistically significant.

The TIR error is dominated by poor-quality images due to prints such asthose shown in the left panel of FIG. 9A. The fingerprint software usedfor this investigation provides a low-quality warning indicator for suchcases. A total of 122 TIR prints were designated as low image quality bythe fingerprint software, which corresponds to 20.3% of all TIR datacollected. Further investigation showed that the TIR image qualityproblems were related to certain study participants, as illustrated inFIG. 14B. This figure plots the fraction of each participant's TIR magesthat were designated as low image quality, which was as large as 90% forone subject. It is clear that certain participants (i.e. numbers 1, 3,7, and 8) were responsible for a majority of the poor TIR images. Avisual examination of the fingers of participants with the largest ratesof low-quality TIR images indicated that these people's skin was notablydry.

The effect of the low-quality TIR images on the resulting biometricperformance can be seen in FIG. 14C, which shows the distributions ofthe match values for both genuine and imposter comparisons. Thesedistributions are shown for two cases. Case A includes all TIR images,as were used to generate the TIR curve in FIG. 14A. Case B wasreanalyzed to remove all instances in which one or both of the imagesbeing matched is designated as low quality. By removing 20.3% of the TIRdata, the performance of the remaining data improved from an EER ofapproximately 20% to 1.8%. The figure shows clearly that removing thelow-quality TIR images results in a significant improvement in thedistribution of the genuine match values, the distribution of theimposter match values changes imperceptibly. None of the direct-imagedata used to generate the corresponding performance curve in FIG. 14Awas designated as low-image quality by the fingerprint software.

For samples in which the corresponding TIR image was not designated aslow quality, the TIR matching score was averaged together with theindividual multispectral scores to produce a composite match value. Incases where the TIR image was designated as low quality, the compositematch value was generated from just the individual multispectral scores,as before. Performing this procedure on the entire dataset yields theROC curve shown in FIG. 14D. There is clear improvement of the direct+TIR data (EER ˜0.4%) versus the multispectral data alone (˜0.7%).

The results of this study indicate that multispectral sensors asdescribed herein are able to collect images from which usefulfingerprint information can be extracted. Moreover, common effects thatseverely affect a conventional TIR sensor have little or no effect on amultispectral sensor. This result, particularly when combined with theresult described above of using multispectral imaging as a powerfulliveness detector, demonstrates a number of useful features ofembodiments of the invention over the prior art.

7. Example

A specific example of a structure that incorporates a sensor asdescribed herein for an embodiment is described in connection with FIGS.15A-15D. In this embodiment, the sensor is comprised by a turnstilehaving the general structure shown in FIG. 15A, the turnstile being ofthe type that may be used to control access by people to different areasin an amusement park, sports arena, or the like. The different areasbetween which the turnstile controls access are denoted as a “paid area”and a “free area” in FIG. 15A. Access is controlled with a dropping-armtripod obstacle 1502. The biometric sensor 1504 may be mounted on thetop surface of the turnstile housing.

A specific structure for the biometric sensor is shown in FIG. 15B, withimaging taking place over a platen 1530 having an ergonomic cover plate;the back of the ergonomic cover plate may act as an optical reference.Direct illumination is provided by illumination sources 1534, some ofwhich include polarizers and others of which do not include polarizers.TIR-image illumination is provided by illumination sources 1532. Nopolarizers are used with the TIR illumination sources. Light is directedto an imager 1538 after encountering the tissue by a turning mirror1536. The imager includes a short-wavelength pass filter and a polarizerthat may be parallel or perpendicular to direct illumination.

In this example, the direct illumination is provided by 18 LEDs made upof two colors and either with or without polarizers. An arrangement forthe 18 LEDs is shown in FIG. 15C, with half of the illumination sourcesproviding green light and half of the illumination sources providingblue light. For convenience, the illumination sources are numbered todefine six different banks, with banks “1” and “3” providing bluepolarized light, banks “2” and “4” providing green polarized light, bank“5” providing blue unpolarized light, and bank “6” providing greenunpolarized light. In this example, the polarizers are linear polarizersand are uniformly aligned.

The TIR illumination sources are shown in FIG. 15D having 8 LEDs made upof two colors without any polarization. These LEDs may also becontrolled as two separate banks, with bank “7” providing blueunpolarized light and bank “8” providing green unpolarized light.

This arrangement may allow an image sequence to be collected with sevenframes: (1) ambient; (2) blue TIR with bank “7”; (3) green TIR with bank“8”; (4) polarized blue direct with banks “1” and “3”; (5) polarizedgreen direct with banks “2” and “4”; (6) unpolarized blue direct withbank “5”; and (7) unpolarized green direct with bank “6.” The imagesequence may be collected in this order or in a different order over aperiod of time on the order of milliseconds. The turnstile may thus beconfigured to perform a biometric assessment of persons attempting tomove from one access area to another using the methods described indetail above.

Thus, having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Accordingly, the above description should notbe taken as limiting the scope of the invention, which is defined in thefollowing claims.

1. A method of performing a biometric measurement on an individual, themethod comprising: illuminating a purported skin site of the individualplaced disposed relative to a platen under a plurality of distinctoptical conditions during a single illumination session, at least one ofthe plurality of distinct optical conditions including illuminating thepurported skin site with light at an angle greater than a critical angledefined by an interface of the platen with an external environment andanother of the distinct optical conditions including illuminating thepurported skin site with light at an angle less than the critical angle;and receiving light scattered from the purported skin site separatelyfor each of multiple of the plurality of distinct optical conditions toderive a multispectral image of the purported skin site.
 2. The methodrecited in claim 1 wherein: portions of the purported skin site are incontact with the platen; and receiving light scattered from thepurported skin site comprises receiving light scattered from positionswhere the light incident on the purported skin site at the angle greaterthan the critical angle coincides with positions where the purportedskin site is in contact with the interface to derive an image of surfacestructure of the purported skin site.
 3. The method recited in claim 1wherein the multiple distinct optical conditions comprise distinctpolarization conditions.
 4. The method recited in claim 1 wherein themultiple distinct optical conditions comprise distinct wavelengths ofillumination light.
 5. The method recited in claim 1 wherein: the platencomprises a thickness of material; and illuminating the purported skinsite with light at the angle greater than the critical angle comprisesreflecting the light multiple times through total internal reflectancewithin the thickness of material before passing through the interface ofthe platen onto the skin site.
 6. The method recited in claim 1 whereinthe light received under each of the multiple optical conditions isreceived by the same light detector.
 7. The method recited in claim 1wherein receiving light scattered from the purported skin site comprisesreceiving light scattered under the at least one of the plurality ofdistinct optical conditions with a first imager and receiving lightscattered under the other of the distinct optical conditions with asecond imager different from the first imager.
 8. The method recited inclaim 7 wherein receiving light scattered under the at least one of theplurality of distinct optical conditions comprises receiving the lightunder a dark-field configuration.
 9. The method recited in claim 7wherein receiving light scattered under the at least one of theplurality of distinct optical conditions comprises receiving the lightunder a bright-field configuration.
 10. The method recited in claim 1wherein a position of the purported skin site is substantially unchangedduring the single illumination session.
 11. A biometric measurementsystem comprising: a platen adapted for placement of a purported skinsite by an individual; an illumination source; a light detector; and acontroller interfaced with the illumination source and the lightdetector, the controller including: instructions to illuminate thepurported skin site under a plurality of distinct optical conditionsduring a single illumination session, at least one of the plurality ofdistinct optical conditions including illuminating the purported skinsite with light at an angle greater than a critical angle defined by aninterface of the platen with an external environment and another of thedistinct optical conditions including illuminating the purported skinsite with light at an angle less than the critical angle; andinstructions to derive a multispectral image of the purported skin sitefrom light received by the light detector after scattering from thepurported skin site for each of multiple of the plurality of distinctoptical conditions.
 12. The biometric measurement system recited inclaim 11 wherein the instructions to derive the multispectral imagecomprise instructions to derive an image of surface structure of theskin site from light received by the light detector after scatteringfrom positions where the light incident on the purported skin site atthe angle greater than the critical angle coincide with positions wherethe skin site is in contact with the interface.
 13. The biometricmeasurement system recited in claim 11 wherein the multiple distinctoptical conditions comprise distinct polarization conditions.
 14. Thebiometric measurement system recited in claim 11 wherein the multipledistinct optical conditions comprise distinct wavelengths ofillumination light.
 15. The biometric measurement system recited inclaim 11 wherein: the platen comprises a first thickness of material;and the illumination source is disposed to illuminate the purported skinsite with light at the angle greater than the critical angle byreflecting the light multiple times through total internal reflectancewithin the first thickness of material before passing through theinterface onto the skin site.
 16. The biometric measurement systemrecited in claim 15 wherein the platen comprises a second thickness ofmaterial through which the light scattered from beneath the surface ofthe purported skin site is propagated before being incident on thepurported skin site.
 17. The biometric measurement system recited inclaim 16 wherein the second thickness of material includes a holedisposed relative to the light detector such that the light scatteredfrom beneath the surface of the purported skin site propagates throughthe hole to the light detector.
 18. The biometric measurement systemrecited in claim 16 wherein the first and second thicknesses of materialare separated by a thickness of the external environment.
 19. Thebiometric measurement system recited in claim 16 wherein theillumination source comprises: a first illumination source disposedwithin the first thickness of material; and a second illumination sourcedisposed within the second thickness of material.
 20. The biometricmeasurement system recited in claim 15 wherein the illumination sourcecomprises an illumination source disposed within the first thickness ofmaterial.
 21. The biometric measurement system recited in claim 11wherein the light detector comprises an optical axis oriented at anangle less than the critical angle.
 22. The biometric measurement systemrecited in claim 21 wherein the optical axis is oriented substantiallynormal to the interface.
 23. The biometric measurement system recited inclaim 11 wherein the controller further includes instructions to performa biometric function with the derived multispectral image and derivedimage of surface structure of the skin site.
 24. The biometricmeasurement system recited in claim 23 wherein the instructions toperform the biometric function comprise instructions to verify anidentity of the individual.
 25. A portable electronic device comprisingthe biometric measurement system recited in claim
 11. 26. A turnstilecomprising the biometric measurement system recited in claim 11.